’Fess up, you’ve got your own blacklist of airlines, hotels, car rental companies and cruise lines you’d do anything to avoid. I know you do because I run the travel industry’s unofficial complaints department, and I get thousands of e-mails each week from angry passengers. Many of these missives end with, “I’ll never do business with your company again!”

So who’s on the list?

US Airways. Most of the complaints I get are about airline service. Delayed and canceled flights, missing baggage, rude flight attendants … the list goes on. Is there one standout? At the moment, it would have to be US Airways. Check out the Transportation Department’s latest numbers. The Tempe, Ariz., airline, underperforms in virtually every category and is the most complained-about carrier.

But hang on. Just a few months ago, US Airways announced a series of what it called “customer service” initiatives designed to “improve reliability and meet customers’ needs.” That included adding airport staff, hiring new customer service agents and being more flexible with some of its policies, particularly for its best customers.

Will it work? Maybe. There’s also this to consider: The airline industry as a whole isn’t scoring well with its customers, with one or two notable exceptions. Major “legacy” carriers such as Northwest Airlines, United Airlines and Delta Air Lines are unlikely to allow US Airways to keep the top spot on the blacklist for very long.

Days Inn. People love to complain about budget hotels, including brands like Days Inn, Econo Lodge and Super 8 Motel. Whenever I get an e-mail about their stay, my first thought is that I’m reading a lost script from the classic TV show “Fawlty Towers.” Then I realize they’re not kidding. It’s difficult to quantify the actual number of complaints about hotels. The federal government doesn’t issue a monthly report card. All I have to go on are my files (which, I admit, is an inexact measure) and what the states — which regulate hotels — have to say.

And Days Inn has kept state governments pretty busy. After 9/11, a Days Inn in New York was penalized for raising room rates by as much as 185 percent in the days following the terrorist attack. And after Hurricane Charlie churned through Florida in 2004, another Days Inn was accused of gouging homeless storm victims. The hotel reportedly paid $70,000 to settle the complaint.

You don’t have to spend a lot of time on sites like Tripadvisor or My3Cents to get an idea of what guests think of many Days Inn hotels. But if you look around, you also see that the hotel chain is hardly alone when it comes to generating complaints. Rich Roberts, a spokesman for Days Inn, says he is unaware of any recent increases in guest gripes, and points out that with 1,862 properties and 150,984 rooms worldwide, his is one of the largest hotel chains in the world, which may account for the volume of letters and calls. “We understand the importance of delivering a positive experience to every guest,” he told me. “Are we perfect? No. But we do our best to avoid repeating mistakes.”

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Thrifty. As with the hotel category, there is no monthly report card for rental cars. I can review my own files, which have more than their fair share of Thrifty complaints. I could look at the latest J.D. Power and Associates ratings which give Thrifty a below-average grade, overall.

But it’s the surcharges that put Thrifty over the top, according to the customers I talk with. And we’re not necessarily talking about the little fees here, either. We’re talking big extras and possibly illegal ones, too. Last year, the former owner of a Thrifty location in Billings, Mont., was convicted in a federal court for conspiring with an auto glass business to overcharge for windshield replacements.

I’ve seen this kind of thing before. A few years ago, I was flooded with complaints about Enterprise. Seems the company was aggressively — and some customers said, fraudulently — pursuing damage claims. Eventually, Enterprise backed down, to the relief of its customers. In other words, the lead car in this race changes often. Yesterday it was Enterprise. Today it might be Thrifty. Tomorrow, who knows?

Princess. Picking a cruise line for this list was the biggest challenge. There’s no way to independently verify the number of complaints about cruises. The Federal Maritime Commission doesn’t issue a regular report on the number of grievances it gets in the same way the Transportation Department publishes an airline report card. And even if it did, I’ve found that cruise complaints tend to be among the most frivolous — long laundry lists of nitpicky items that don’t always rise to the level of legitimacy.

It isn’t even that Princess generates more grievances than the others. (I asked Princess spokeswoman Julie Benson, and she said the cruise line hadn’t experienced any recent surge in complaints.) It’s that when passengers do complain, the company’s attitude often seems to be dismissive. And that doesn’t exactly encourage customers to book another sailing on The Love Boat. My colleague Anita Potter documents the company’s apparent indifference in a recent column in which a passenger is wrongfully denied boarding and then ignored when she asks for a refund of her expenses. Princess is remarkably consistent. Even my requests for assistance on behalf of other travelers are usually met with a “we’ll look into it” followed by a long silence. One reader recently referred to its passenger relations department as a fortress. That’s a good way to put it.

Should you avoid these companies, too? In a perfect world, travelers would be able to boycott companies that gave them bad services. In reality, they can’t. You don’t always have a choice in airline, hotel, car rental agency or even cruise line. But that shouldn’t stop you from keeping score.

I’ll be taking a close look at what makes the travel business tick in this column that appears here every Monday. Your comments are always welcome, and if you can’t get enough of my column, drop by my blog for daily insights into the world of travel.